Zmanim (halachic times of day) calculation
By default we follow
chabad.org1 zmanim standards. That is, we follow the
Alter Rebbe (aka Baal HaTanya) for times of day unless you choose otherwise.
The time for end of Shabbat and Yom Tov is shown according to the time for 3 small
stars. On weekdays, however, we use the universal standard for nightfall of three
medium stars. The Zmanim page will list both times on Shabbat and Yom Tov. Note that
the later time must be observed for activities that are forbidden on Shabbat. See
below for the methodology details.
Specifying the opinion you follow
The Zmanim page lets you specify which halachic opinion to use for calculating the
daytime hours (sha'ah zmanit). This choice affects most daytime
zmanim: latest time for Shema, latest time for Shacharit, Mincha Gedola, Mincha
Ketana, and Plag HaMincha. Three general opinions are available:
Alter Rebbe (default) — Follows R' Shneur Zalman of Liadi in his
Siddur2. Halachic hours are measured from
the amiti (astronomical) sunrise to amiti sunset — the moment the geometric center
of the sun's disk crosses the horizon, ignoring refraction. End of Shabbat is set
to when the sky darkens to 8.5° below the horizon. This is the standard for the
Chabad community.
GR"A (Vilna Gaon) — Halachic hours are measured from visible
sunrise (when the sun's upper edge first appears) to visible sunset. End of Shabbat
is set to when three medium-sized stars are visible, approximately 6° below the
horizon. This is the majority opinion followed by most Ashkenazi and Sephardi
communities. Source:
Biur HaGr"a OC 459:23.
Magen Avraham — Halachic hours are measured from dawn (72 minutes
before sunrise, or when the sun is 16.1° below the horizon) to nightfall (72 minutes
after sunset). Because the "day" is longer, halachic hours are longer, and
every daytime zman falls earlier than with the GR"A. Some communities follow
this stringently for Shema. At very high latitudes where the sun never reaches 16.1°
below the horizon, halachic hours are computed from halachic midnight instead — see
Magen Avraham at high latitudes.
Note: Magen Avraham's version of the 72-minute nightfall is used only for
calculating the halachic hour and the parts of the day; it is not used for any other
halachic purpose18.
Additional zmanim (opt-in)
These reference rows are hidden by default and can be turned on in the gear menu:
- Dawn — Rav Avraham Chaim Naeh (−26°)
- Dawn — Sefer Bein Hashmashot (−19.8°)
- Misheyakir Sefer Eretz Israel and Kaf haChaim (−11.5°)
- Misheyakir — Sefer Nivreshet (−11.8°)
- Nightfall — Rabbeinu Tam (72 minutes after sunset)
- Nightfall — Sefer Bein Hashmashot / Igrot Moshe (−8.5°)
- Nightfall — Melamed Lehoil (−7.083°)
Zmanim definitions
Times are computed from your geographic location's astronomical events. Angles
given below refer to the sun's depression below the horizon.
Sunrise and sunset
Visible sunrise (hanetz hachama) — The moment the top of the sun
first appears above the horizon: sun's upper edge at −0.833° (accounting for average
atmospheric refraction and the sun's semi-diameter).
Visible sunset (shkiat hachama) — Same 0.833° threshold at the end
of the day: sun's upper edge disappearing on the western horizon.
Amiti sunrise / amiti sunset (Baal HaTanya) — The astronomical
event: the geometric center of the sun's disk at −1.583°, i.e., disregarding
refraction and taking the true horizon. This is what the Alter Rebbe treats as
sunrise/sunset for the purpose of the halachic day. Source:
Seder Hachnasat Shabbat, Siddur Torah Ohr2.
Halachic hour (sha'ah zmanit)
A halachic hour is one-twelfth of the halachic day. What counts as the "day"
depends on the opinion followed:
- GR"A — visible sunrise to visible sunset. Source:
Biur HaGr"a OC 459:23.
- Magen Avraham — dawn (72 minutes / 16.1°
before sunrise) to nightfall (72 minutes / 16.1° after sunset). Source:
Magen Avraham OC 58:14.
Note that this nightfall time is not used for any other purpose.
- Alter Rebbe — amiti sunrise to amiti sunset.
Source:
Seder Hachnasat Shabbat, Siddur Torah Ohr2.
Dawn (alot hashachar)
The point at which the eastern sky begins to lighten. The site supports several
opinions:
- GR"A / Magen Avraham (72 minutes) — sun at −16.1° below
the horizon. Traditionally rendered as "72 minutes before sunrise"; the
site uses the angle so results scale correctly at all latitudes and times of year.
- Alter Rebbe (Chabad) — sun at −16.9°. Source: Shulchan Aruch
HaRav OC 89:1.
- Rav Avraham Chaim Naeh — 120 minutes, sun at −26°; used for
certain stringencies such as the beginning of fasts. Source:
Shiurei Tzion5.
Note that even at moderately high latitudes like Montreal or Northern Europe this
zman will not show in the summer, because the sun does not descend below that
angle.
- Sefer Bein Hashmashot — sun at −19.8°, corresponding to the
90-minute figure derived by R' Yechiel Michel Tukachinsky. Source:
Sefer Bein Hashmashot6.
Misheyakir (earliest talit and tefillin)
The point at which one can distinguish the blue thread of the tzitzit from the
white. This is the earliest time to put on talit and tefillin.
- chabad.org default — sun at −10.2°. Several authorities
place this moment at 45 equinoctial minutes before sunrise.
- Sefer Eretz Israel — sun at −11.5°. Source:
Sefer Eretz Israel7.
See also
Kaf haChaim 18:188.
- Sefer Nivreshet — 52 minutes before sunrise (sun at −11.8°). Source:
Sefer Nivreshet9
on the earliest time for talit and tefillin.
Latest Shema (sof zman kri'at shema)
The end of the third halachic hour from the start of the day (per the selected
opinion). Source:
Mishna Berachot 1:210,
per the ruling of R' Yehoshua that we follow;
Shulchan Aruch HaRav OC 58:311.
Latest Shacharit (sof zman tefila)
The end of the fourth halachic hour. Source:
Berachot 26a12 — up to four
hours the tefilla is le-chatchila.
Erev Pesach — latest to eat and burn chometz
- Latest time to eat chometz — end of the fourth halachic hour.
- Burn chometz before — end of the fifth halachic hour.
Chatzot — halachic noon and midnight
Chatzot hayom (noon) — the midpoint between sunrise and sunset.
Chatzot halayla (midnight) — 12 halachic hours after noon, i.e.,
the midpoint between sunset and the following sunrise.
Mincha Gedola
6.5 halachic hours from the start of the day. The earliest time one may pray Mincha.
Mincha Ketana
9.5 halachic hours from the start of the day. Preferred earliest time for Mincha
le-chatchila.
Plag HaMincha
10.75 halachic hours from the start of the day. The earliest time one may accept
Shabbat or daven Ma'ariv (per certain opinions).
Sunset (shkia)
Visible sunset — the sun's upper edge disappearing on the western horizon
(−0.833°). This begins bein hashmashot, the disputed period between day and night.
Nightfall (tzait hakochavim)
The end of bein hashmashot — the halachic transition from day to night. Several
opinions:
- Three medium stars (visible, GR"A / MA display) — sun at
−6°. Source:
Shulchan Aruch OC 293:213.
- Alter Rebbe / Igrot Moshe / Sefer Bein Hashmashot — sun at
−8.5°. Sources: Shulchan Aruch HaRav OC 293:2;
Igrot Moshe14
OC 4:62;
Sefer Bein Hashmashot15.
R' Moshe Feinstein writes that the longest
bein hashmashot in New York is 50 minutes, corresponding to 43 minutes in Jerusalem
(per Sefer Bein Hashmashot), which in turn is equivalent to 36 minutes in Jerusalem
at the equinox — the sun depression that yields the −8.5° angle used here.
- Rabbeinu Tam — 72 minutes after visible sunset (fixed
equinoctial minutes), following Tosafot's calculation of four Roman miles at 18
minutes each. Source: Tosafot Shabbat 35a "trei tarti gavni".
Held stringently for the end of Shabbat by some communities. The site displays it
as an optional reference row and suppresses it when it would fall earlier than the
primary tzait at extreme latitudes.
- Melamed Lehoil — sun at −7.083°. Source: Responsa
Melamed Lehoil16
OC 30 (R' Dovid Zvi Hoffmann), discussing end of Shabbat in European latitudes.
Candle lighting
- Erev Shabbat and Erev Yom Tov — 18 minutes before sunset by
common custom (Jerusalem: 40 minutes). Source:
Shulchan Aruch OC 261:217
establishes tosefet Shabbat; the specific 18/40 minute intervals are minhag.
- Day 1 of a two-day Yom Tov, when Day 2 is not on Shabbat and begins
after sunset, or when Yom Tov occurs after Shabbat — Candles are lit
from an existing flame after tzait hakochavim, using three small stars
(−8.5°).
End of Shabbat and Yom Tov
Nightfall with three small stars, 36 minutes at −8.5°. This is the same for all
major opinions. Source:
Shulchan Aruch OC 293:213.
When the Rabbeinu Tam row is enabled, it appears alongside as a stringent
alternative. Rabbeinu Tam nightfall might not be displayed in high latitudes where
it occurs before the time of three small stars.
Magen Avraham at high latitudes
At high enough latitudes (roughly above 50° in the summer/winter extremes) the sun
never dips 16.1° below the horizon, so the Magen Avraham dawn does not occur. When
this happens, the site sets the halachic hour to two clock hours (equivalent to
treating the halachic day as 24 hours long) and reckons all daytime zmanim from
halachic midnight. Times computed this way are marked with an asterisk (*) and carry
a footnote advising to consult a competent rav before relying on them.
References
Date calculations
Birthday
A Hebrew birthday is generally observed on the same Hebrew date each year. Two
exceptions:
- Born in Adar of a non-leap year — in a leap year the birthday
is observed in Adar II.
- Born on 30 Cheshvan or 30 Kislev — in years where that month
has only 29 days, the birthday rolls forward to 1 Kislev (respectively 1 Tevet),
which is Rosh Chodesh.
Yahrzeit
The yahrzeit is generally observed on the same Hebrew date each year. The
exceptions differ from the birthday rule:
- Passed away in Adar of a non-leap year — most authorities today
observe the yahrzeit in Adar I of a leap year, following the Rama; this is what we
compute. The Shulchan Aruch and some Sephardic authorities rule Adar II. In the
first year after passing, however, Adar I is preferred by many of those opinions
who otherwise rule in favor of Adar II, since exactly 12 months must have elapsed
(Magen Avraham there). See
Shulchan Aruch OC 568:76.
- Passed away in Adar I of a leap year — subsequent leap years:
Adar I; non-leap years: the sole Adar.
- Passed away in Adar II of a leap year — subsequent leap years:
Adar II; non-leap years: the sole Adar.
- Passed away on 30 Cheshvan or 30 Kislev — the rule depends on what
happened in the first year after death:
- If that year had a 30th of the same month (a full month), then in
future years with only 29 days the yahrzeit is observed on Rosh Chodesh
(1 Kislev or 1 Tevet respectively) — same as the birthday rule.
- If that year did not have a 30th (the month was deficient), the
yahrzeit is observed on the last day of that month every year — 29 in
deficient years, 30 when the month is full.
See also Kitzur Shulchan Aruch 2211.
Other recurring events
Bar / Bat mitzvah, wedding anniversaries and other recurring Hebrew-date events
follow the birthday rule above.
The 19-year cycle
The Hebrew calendar reconciles the lunar month with the solar year by inserting a
leap month (Adar I) in 7 of every 19 years. Over the full cycle, 19 solar years
equal almost exactly 235 lunar months (12 × 12 + 7 × 13), so months and seasons
stay aligned.
Within each 19-year cycle, the leap years are years 3, 6, 8, 11, 14, 17
and 19. This is remembered by the mnemonic
גו״ח אדז״ט, where each Hebrew letter is its
numeric position modulo 10.
The current Hebrew year's position in the cycle is:
((year − 1) mod 19) + 1
Source: Rambam, Hilchot Kiddush HaChodesh 6:112.
Computing the day of Rosh Hashana
The molad (new moon) of Rosh Hashana is computed from the traditional constants:
the mean synodic month is 29 days 12 hours 793 chalakim (1 chelek = 1/1080
hour ≈ 3⅓ seconds). Molad Tishrei of any year is projected from
Molad Beharad — the reference molad of Nisan year 1, which fell on Monday
at 5 hours 204 chalakim after 6 PM Sunday. Note that this molad (and almost the
entire year 1) is before creation according to the tradition that the world was
created in Tishrei (Rabbi Eliezer in
Rosh Hashana 10b7), and is
sometimes called molad tohu.
Once the molad is known, Rosh Hashana can be postponed by up to two days via the
four dechiyot (postponement rules):
- Molad zaken (מולד זקן) — if the molad
falls at noon or later, Rosh Hashana is pushed to the next day.
- Lo ADU rosh (לא אד״ו ראש) — Rosh Hashana
never falls on Sunday, Wednesday or Friday. If the molad (or the day after molad
zaken) lands on one of these, push forward one day.
- Gatarad (גטר״ד —
ג=Tue, ט=9h,
ר״ד=204ch) — in a non-leap year, if the molad falls on
Tuesday at 9 hours 204 chalakim or later, Rosh Hashana is pushed to Thursday
(via lo ADU rosh).
- Betutakpat (בט״ו תקפ״ט —
ב=Mon, ט״ו=15h,
תקפ״ט=589ch) — in the year immediately after a leap year,
if the molad falls on Monday at 15 hours 589 chalakim or later, Rosh Hashana is
pushed to Tuesday.
Source: Mishneh Torah — Hilchot Kiddush HaChodesh, chapters 6–82.
Parshiyot
The schedule of weekly Torah readings is governed by the rules set out in
Tur, Orach Chaim 4283. There are
14 year types, determined by: (1) the day of the week of Rosh Hashana, (2) whether
Cheshvan and Kislev have their standard number of days, and (3) the day of the week
of the first day of Pesach. The rules for merging and splitting certain parsha pairs
were codified by R. Chaim Mordechai haCohen Brecher and are printed in the back of
some Chumash editions
(see table5).
Each year type is encoded with three Hebrew letters: the day of Rosh Hashana
(ב Mon · ג Tue · ה Thu · ז Sat), the year length (ח deficient · כ regular · ש
complete), and the day of the first day of Pesach. This code appears in Chumashim
and Tikun Korim.
In some years certain readings in Eretz Israel differ from the Diaspora because a
second day of Yom Tov displaces a reading in the Diaspora. The Parshiot page shows
both readings.
Tekufot and Birkat HaChama
Tekufot are the four astronomical seasons: Tekufat Nisan (spring equinox), Tekufat
Tammuz (summer solstice), Tekufat Tishrei (autumnal equinox) and Tekufat Tevet
(winter solstice). Two calculations are traditional:
- Tekufat Shmuel — a simple year of exactly 365¼ days, giving
91⅜ days per season. This is what governs practical halachic reckoning today
(Tal uMatar in the Diaspora, Birkat HaChama). Source:
Eruvin 56a4;
Rambam, Kiddush HaChodesh 9:42.
- Tekufat Rav Ada — matches the mean solar year underlying the
molad calculation (365 days, 5h, 55m, 25 25⁄57 sec); more accurate but retained
mostly for theoretical reference. Source:
Rambam, Kiddush HaChodesh 10:12.
Birkat HaChama (blessing on the sun) is recited every 28 years,
on the morning after Tekufat Nisan (per Shmuel) falls on Tuesday evening /
Wednesday sunrise. The last occurrence was Wednesday 8 April 2009; the next is
8 April 2037.
This is because, according to Shmuel, Tekufat Nisan advances by 1¼ days in the
weekly cycle every year, so once in 28 years it falls on Tuesday at 6 AM — the
hour the sun was traditionally created. There is no associated astronomical
event; it is the halachically accepted tradition.
Source: Mishneh Torah — Hilchot Kiddush HaChodesh, chapter 102.
References